The present invention relates to dual-polarized tracking antennas, and more particularly to reducing cross-polarized signals in the vicinity of such antennas.
Dual-polarized tracking antennas have four difference ports (.DELTA.)-one pair for each polarization. FIG. 1 illustrates in schematic form a dual-polarized monopulsetype tracking antenna in which the polarizations are horizontal (H) and vertical (V). While any pair of orthogonal polarization may be used, the six ports in this example are:
.SIGMA.(H)-sum (horizontally Polarized) PA1 .DELTA..sub.a (H)-azimuth difference (horizontally polarized) PA1 .DELTA..sub.a (H)-elevation difference (horizontally polarized) PA1 .SIGMA.(V)-sum (vertically polarized) PA1 .DELTA..sub.a (V)-azimuth difference (vertically polarized) PA1 .DELTA..sub.e (V)-elevation difference (vertically polarized) PA1 When a single polarization is used, a set of three ports is chosen, either the horizontal or vertical.
Radiation patterns when these ports are energized, if, for example, the polarizations are horizontal and vertical, are illustrated in FIGS. 2(a)-2(d). The solid patterns represent the desired co-polarized radiation and the dashed patterns represent the undesired cross-polarized radiation. The cross-polarized lobes usually are about 10dB to 20dB lower than the co-polarized lobes for a conventional antenna. The cross-polarization results in the antenna's susceptibility to tracking errors and to breaklocks.
Prior art techniques to reduce the cross-polarized radiation in the vicinity of the axis of a dual-polarized tracking antenna involved choosing antenna and feed geometries, but satisfactory performance could not always be achieved and some degradation of performance had to be accepted. Nulling techniques for communication systems use a single dual-polarized antenna beam to achieve their nulls and hence they are effective over only a narrow angular region of the beam and these techniques are not applicable to cross-polarization in the difference patterns of tracking antennas.
Variable-ratio microwave power dividers, utilizing two spaced-apart 90 degree hybrids directionally coupling two microwave channels and an adjustible phase shifting device disposed between the two 90 degree hybrids, have been known in the art for many years. For example, such a variable power divider is described in a paper entitled "A Variable-ratio Microwave Power Divider and Multiplexer", by W. L. Teeter and K. R. Bushore, IRE Transactions on Microwave Theory and Techniques, October, 1957, pages 227-229. In this power divider, an incoming microwave signal may be directed solely to either of two outputs or may be divided in any disired ratio between the two outputs by adjustment of the phase shifting device.